Merge tests and fixes from stale PRs

[mielvds]

This PR attempts to integrate tests and fix from other PRs. Some of them are very old and are not mergeable anymore. However, most of these issues still persist and to not let this work go to waste, they are ported here:

Already includes tests from or for issues:

• It is not always relevant to silently ignore unrecognized properties #50
• IRI expansion with missing @base does not conform to RFC 3986 #187
• Nested "@id" elements are not processed correctly in framing #149
• Fixed bugs in embed and remote frame context handling. #31
• Dates from schema.org are not compacted correctly. #59
• String xsd:double value crash toRdf() #202

Already includes fix from or for:

• Proposed fix for issue #50 #51
• IRI expansion with missing @base does not conform to RFC 3986 #187
• Nested "@id" elements are not processed correctly in framing #149
• Fixed bugs in embed and remote frame context handling. #31
• Always try to add a default @type and @language #60 --> introduces regressions, so no included
• String xsd:double value crash toRdf() #202