$convert between XML and JSON is inconsistent

[gabriel0316]

Describe the bug When using the $convert operation the transformation of a resource turns out to be inconsistent.

Environment 4.7.1

To Reproduce Steps to reproduce the behavior - XML to JSON:

1. Take the following resource as XML and pass it on to the $convert operation:

   <CodeSystem xmlns="http://hl7.org/fhir">
       <name value="test-codesystem"/>
       <title value="Test Code System"/>
       <status value="active"/>
       <date value="2016-11-24"/>
       <description value="XXX&#10;&#10;XXX"/>
       <content value="complete"/>
       <concept>
           <code value="0"/>
           <display value="TEST CODE"/>
       </concept>
   </CodeSystem>
   

2. The resulting JSON representation of the resource will be:

   {
       "resourceType": "CodeSystem",
       "name": "test-codesystem",
       "title": "Test Code System",
       "status": "active",
       "date": "2016-11-24",
       "description": "XXX\n\nXXX",
       "content": "complete",
       "concept": [
           {
               "code": "0",
               "display": "TEST CODE"
           }
       ]
   }
   

Steps to reproduce the behavior - JSON to XML:

1. Take the following resource as JSON and pass it on to the $convert operation:

   {
       "resourceType": "CodeSystem",
       "name": "test-codesystem",
       "title": "Test Code System",
       "status": "active",
       "date": "2016-11-24",
       "description": "XXX\n\nXXX",
       "content": "complete",
       "concept": [
           {
               "code": "0",
               "display": "TEST CODE"
           }
       ]
   }
   

2. The resulting XML representation of the resource will be:

   <CodeSystem xmlns="http://hl7.org/fhir">
       <name value="test-codesystem"/>
       <title value="Test Code System"/>
       <status value="active"/>
       <date value="2016-11-24"/>
       <description value="XXX
   
   XXX"/>
       <content value="complete"/>
       <concept>
           <code value="0"/>
           <display value="TEST CODE"/>
       </concept>
   </CodeSystem>
   

Expected behavior When converting from JSON to XML the \n should be transformed to 
. Then it would be possible to convert between XML and JSON consistently.

[lmsurpre]

https://www.w3.org/TR/REC-xml/#AVNormalize has me agreeing this could be an issue. Based on my reading of that, an XML parser would parse

<description value="XXX&#10;&#10;XXX"/>

to an attribute value of

XXX

XXX

whereas it would parse

<description value="XXX

XXX"/>

to an attribute value of

XXX XXX

We don't do any special $convert logic, we just read the resource and then write it back out using XMLStreamWriter.writeAttribute.

Unfortunately, I don't see any supported configuration for preserving the newlines. Its basically the same issue that is reported here: https://stackoverflow.com/questions/8331364/how-to-preserve-whitespace-in-attributes-when-using-xmlstreamwriter

I'd lean toward moving this to the icebox and reporting a related issue upstream. How important is this one to you?

[lmsurpre]

I added a test at https://github.com/LinuxForHealth/FHIR/pull/3816/files#diff-8ec2e5b4061497aaf09998b194bb8e294ab785f38c512b5d64119ed4940e7463R23-R38 to demonstrate the root of the issue which has nothing to do with JSON and everything to do with XML.