wt- and xml version-parameter fix in connection classes

[tobias-graf-p]

I use the SolrConnection.cs class directly for some features currently not supported by SolrNet (e.g. Suggester). While doing so, I run into the situation, that SolrNet sent requests like the following to Solr server:

http://...:8983/suggest?...&wt=json&wt=xml&version=2.2

I set the wt param to "json", but both xml and json were included in the request url. In addition the xml version was included.

I found out, that issue #508 and pr #509 already addressed the first issue but fixed it only in SolrConnection class (but not in SolrAutoConnection class).

This pr is about the following:

• only fallback to wt=xml if wt was not set by the caller also in AutoSolrConnection
• only set version if wt is set to xml since version specifies the xml protocol version (see Solr docs)

What I am not sure about:

• Is is correct, that the fix from pr #509 also should be applied in AutoSolrConnection class?
• Should the same logic also be applied in SolrPostConnection class (see line 65)? This is not included in this pull request.
• Should tests be modified or completed? (WithWtJson_Get() expects wt=json and version=2.2 and run through in my branch using my modifications but I didn't understand why.)