StaticContentServer middleware conforms closer to spec for byte range requests.

This commit makes the following changes:
1. If there are syntactic errors in the Range header, it is ignored and
the full content is returned. This conforms to the spec.
2. In case of multiple ranges in the header, the full content is returned.
This behavior differs from the spec which says that a multipart response
should be returned.

PLAT-104
Added tests for parse_range_header() in contentserver.

PLAT-104