Time-dimension in REST style WMTS-URLs

This pull requests improves the WMTS REST URL scheme in two ways:

 * a bug was fixed that prohibited the use of time-dimensions in WMTS REST style URLs.
 * The "double hyphen" (i.e: '--') was allowed as a separator between start and end time in time intervals. This enables the usage of time intervals in REST style URLs. The double hyphen mentioned in a note in clause 4.4.2 of ISO 8601.