Fixes problem whereby QueryArgs were not populated in generated URL.

Hadn't tested generation of AppLocalUrls containing QueryArg parameters.
Oops. Adds test, and fixes fault.
Changes behaviour of NullableComponent to allow null replacement string.

The 'nullValueString' of NullableComponent may itself be (string)null,
which allows it, when used in a QueryArg, to cause the argument to be
omitted when the value is null.

Adds unit tests to cover the usage in path components and query arguments.
Adds copyright headers to test files.
Upversioned to 2.1.0

* Query parameters are generated properly for URLs
* 'null' query parameters may be omitted from querystring