Retrofit template: return body string directly when failed to parse response body as JSON

[xhh]

When the deserialization fails due to JsonParseException and the expected type is String, then just return the body string.

This PR also fixes the unit test of UserApi#loginUser, which throws exception:
retrofit.converter.ConversionException: com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 9 path $
See the comment of the Retrofit template's author here

[xhh]

integration test is good

Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
Running io.swagger.petstore.test.PetApiTest
Tests run: 7, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 25.702 sec
Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
Running io.swagger.petstore.test.StoreApiTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 5.015 sec
Concurrency config is parallel='methods', perCoreThreadCount=true, threadCount=2, useUnlimitedThreads=false
Running io.swagger.petstore.test.UserApiTest
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 6.215 sec

Results :

Tests run: 15, Failures: 0, Errors: 0, Skipped: 0