Compliance: Improved HTTP Range header field validation.

Compliance: Improved HTTP Range header field validation.

1) Improved HttpHdrRangeSpec::parseInit() to parse syntactically valid
range specs:

  * Suffix ranges with 0 length (i.e. -0) are syntactically valid.

  * Check that last-byte-pos is greater than or equal to first-byte-pos.

After the change, HttpHdrRangeSpec::parseInit() successfully parses suffix
ranges with 0 length. They were rejected before. RFC 2616 section 14.35.1 says
such range specs are syntactically valid but unsatisfiable. Thus, we should
ignore the range spec itself, but not the whole range header. These range
specs will be rejected later, during canonization.

2) In HttpHdrRangeSpec::parseInit(), ignore the whole range header if one of
range specs is syntactically invalid (i.e. range spec parsing fails).

Co-Advisor test case: test_clause/rfc2616/invalidRange