Handling of expansion of empty `@base`.

[davidlehn]

Current code, after RFC3986 fixes, handles empty @base in an unexpected way. See example in json-ld/json-ld.org#535.

• Determine if undefined empty @base behavior is ok.
• Add json-ld.org test suite test for behavior one way or another?
• Add local test so code doesn't change the "undefined" behavior?
• Raise error and suggest using null?
• Update lib code that is setting base to "" if something else (null?) is more appropriate.

[davidlehn]

Looks like empty and relative values are allowed for @base. Code needs to be updated and hopefully the main test suite will grow tests for this.

[davidlehn]

Fixed and test suite tests added. Released in 0.5.17.