Add null checks for GsonBuilder methods

[Marcono1234]

Currently most GsonBuilder methods do not check for null. This can lead to NullPointerExceptions being thrown at completely unrelated places when the created Gson instance is used. (Might be the reason for #1578.)

Have not added a null check to setDateFormat(String pattern) because it looks like the pattern can be null (at least that is the default value).

Also resolves #1528

[eamonnmcmanus]

Thanks! This looks great.

Separately, perhaps we should get rid of $Gson$Preconditions.checkNotNull entirely and replace it with Objects.requireNonNull now that we are require Java 7.

[Marcono1234]

Separately, perhaps we should get rid of $Gson$Preconditions.checkNotNull entirely and replace it with Objects.requireNonNull now that we are require Java 7.

Good point! Have created #2180 for that. (I hope you weren't working on this as well already.)