Fix wrong cc,bcc,to field names

[bob983]

Description

@teinemaa found out that cc, bcc and to fields are not serialised and parsed properly. He proposed a fix in #18.

This PR extends the fix by providing tests and keeping the Conversation and AbstractThread fields in place and only renaming the JSON fields via GSON @SerializedName annotation.

[vrto]

One general observation: Not sure how long does it take to execute these tests, but I'd perhaps consider merging the test to test all cc/bcc/to fields ... up to you!

[vrto]

Otherwise 👍 :)

[bob983]

It takes 6 seconds to run them all. If possible I prefer small tests with one assert - it actually was done as one big test but I decided to split it into several small tests.

[vrto]

Maybe it doesn't sound like an issue now, but it can get out of control easily. It happened to us on Core-API - at certain point we grouped similar assertions and improved exec times by couple of minutes :)

[bob983]

I understand your point, but I see it as premature optimisation now - the tests only program WireMock, that is already started and call java code (that requests data from WireMock).

[vrto]

Roger that.