Clearing methods for parameters and setBaseUri(URI) method in reqSpecBuilder

Now you can't manage already added parameters on request spec builder, so for changing only one parameter you should rewrite whole builder from start. Clearing parameters can be useful if you use api-specific wrapper around this builder. 
With this PR it will be possible to store only builder, without maps for each parameter type in such wrappers.

with `setBaseUri(URI)` method you shouldn't do `someURI.toString()` in every call