GEOS-6688 - WCS 1.1.1 GetCoverage request fails

Edited the WCS 1.1.1 class to let cubic (as bicubic) and linear (as
bilinear) be used as valid interpolation methods. Those were disabled in
code and GeoServer would serve an Exception when trying to access those
interpolations.

I've also added prelimenary support for bicubic and bilinear as keywords
and made a very small performance improvement on GetCoverage requests.
[GEOS-6688] WCS 1.1.1 GetCoverage request fails

This commit adds the needed tests for the GetCoverage request.

It also fixes the WCSTestSupport class and enables all the tests in the
GetCoverageReaderTest class, which were disabled.
[GEOS-6688] WCS 1.1.1 GetCoverage request fails test addition
Merge pull request #765 from Geodan/GEOS-6688

[GEOS-6688] WCS 1.1.1 GetCoverage request fails...
Merge branch 'master' of git://github.com/geoserver/geoserver