New accessor "hasParameter(key)"

Hello,

first let me say, that i think your UrlBuilder is a really good piece of software. Small and simple to use. 

I'm planning on using the UrlBuilder in my project and i missed the above mentioned accessor method for parameters already present on a given instance of an UrlBuilder.

Please pardon the three whitespace changes on the indentation of the method signatures - they have been done by the autoformatter of intelliJ. :)

I allowed myself to create a new test class as i was unsure of where to put the simple test.


Regards,
Torben