formatTypeMapping fails with primitive types

[HectorBst]

Hi,

Use the formatTypeMapping config with primitive types throw an exception at generation.

For instance:

<formatTypeMapping>
	<int32>int</int32>
	<int64>long</int64>
</formatTypeMapping>

Will result to:
[ERROR] Failed to execute goal org.jsonschema2pojo:jsonschema2pojo-maven-plugin:1.0.2:generate (default) on project Test: Execution default of goal org.jsonschema2pojo:jsonschema2pojo-maven-plugin:1.0.2:generate failed: int is a primitive

The error seems to come from the apply method of the FormatRule class, which should not call JCodeModel.ref with primitive types.