Add methods to allow for better configuration of header values in WicketTester

At current, there seems to be no way to override header values that WicketTester uses. Specifically, it's using a very old user-agent string for requests. For applications that disallow access to unsupported browsers, this requires adding testing run-state checking into production code to keep builds from failing due to test-case failure.

At a minimum, please add #setHeader(name, value) to allow overriding of header values to MockHttpServletRequest. There are probably other areas that could be improved to allow better configurability, but this would cover a good portion of it to start with.

(I assume this also affects 1.4.17 and 1.5.*, but I haven't verified that, so I left it off of the affected versions)