RFC 6868 - " and \n in parameters

[niccokunzmann]

RFC 6868 adds these parsing rules:

When generating iCalendar or vCard parameter values, the following
apply:
o formatted text line breaks are encoded into ^n (U+005E, U+006E)
o the ^ character (U+005E) is encoded into ^^ (U+005E, U+005E)
o the " character (U+0022) is encoded into ^' (U+005E, U+0027)
When parsing iCalendar or vCard parameter values, the following
apply:
o the character sequence ^n (U+005E, U+006E) is decoded into an
appropriate formatted line break according to the type of system
being used
o the character sequence ^^ (U+005E, U+005E) is decoded into the ^
character (U+005E)
o the character sequence ^' (U+005E, U+0027) is decoded into the "
character (U+0022)
o if a ^ (U+005E) character is followed by any character other than
the ones above, parsers MUST leave both the ^ and the following
character in place

With a few test cases, we can support them.

To close this issue:

• write tests to make sure the input is parsed properly - in a test_rfc_6868.py fie
• submit the tests in a PR
• fix the tests
• add documentation that we now also support this RFC (usage.rst)
• add yourself as a contributor

See also:

• Claiming compatibility to other RFCs/updates of RFC5545 #589
• Support DQUOTE character in property parameter values #219
• https://www.rfc-editor.org/rfc/rfc6868.html