= http: support parsing of full ISO8601 strings

The new implementation of spray.http.DateTime.fromIsoDateTimeString now
also supports strings in format `yyyy-mm-ddThh:mm:ss.123456Z`.
= http: make comment more specific
= http: fix DateTime.fromIsoDateTimeString

In addition fixed the incorrect corresponding test cases.
= http: replace forall closure

Increases performance. This solution has been adviced by @sirthias.
= http: restrict to `yyyy-mm-ddThh:mm:ss[.SSSZ]`
= http: add comments on milliseconds discarding
Merge pull request #816 from choffmeister/815-support-full-iso-datetime-string

= http: support parsing of full ISO8601 strings