ISSUE-238 : Make AvroSchemaProvider honour string representations when canonicalising

[raju-saravanan]

This pull request addresses the issue: #238

[satishd]

@raju-saravanan After thinking through the reported issue more , I realized that these changes may not be really needed except for having validation of avro.java.string with right values. Put a comment in issue #238 and want to hear others opinions.