Improve null handling when adding custom header
 - Fixed Eclipse warnings
 - Jackson representation now ignore empty value (null or empty collections)
   by default in the JSON/XML/CSV/YAML produced, leading to more compact
   representations.
 - Added unit test for exceptions serialization with Jackson.
 - Improved Jackson exception test case
Merge pull request #972 from boillodmanuel/issue971-improve-null-handling

Improve null handling when adding custom header
 - deprecated static HttpRequest.addHeader method in engine to favor usage of new Message#getHeaders() method
move method HttpClientHelper.getSocketConnectTimeoutMs() to parent class
fix edition GWT
fix GWT edition: add StringWriter.close() in gwt version
Merge branch 'master' into apispark-2.3
issue #216 - Remove HeaderName. Use simple String instead.
typo and suggestion from @addenie
issue #216 - Follow CORS steps
Update CORS api