* Add the Truth dependency
* Port Junit test to Truth in the package `com.google.gson` of the module `gson`
* Replace the `assertThat(e.getMessage()).isEqualTo(...)"` with `assertThat(e).hasMessageThat().isEqualTo(...)`
* Minor fixes
Previously when a GsonBuilder had created a Gson instance and was afterwards
reused and different type adapters were added, new GsonBuilder instances
obtained from the previous Gson instance through Gson.newBuilder() would have
been affected by the GsonBuilder changes.
This commit fixes this and additionally adds some more unit tests.
