Correctly format contig output lines from writer, making output VCFs compatible with GATK. Fixes #74

This is a fix for issue 74:

https://github.com/jamescasbon/PyVCF/issues/74

The raw dictionary output of PyVCF on contig lines causes errors if you process files downstream with GATK. This pull request detects and correctly formats "##contig" metadata. Includes an added test case to avoid regression.