Ignore 'Language' header in PO-templates

[sinoroc]

The tools 'xgettext' from GNU gettext and 'pot-create' from lingua both produce PO-template files containing a 'Language' header with an empty string as value.

Babel has issues creating catalogs out of such templates.

It wants to parse the PO-template as a normal PO-catalog using the locale of the target catalog, because it helps producing the correct plurals. But it overwrites this locale when parsing the 'Language' header and then it can not produce a valid catalog.

Ignoring the 'Language' header when it is known that the file being processed is a template avoids this issue.

GitHub: fixes https://github.com/python-babel/babel/issues/665

[codecov-io]

Codecov Report

Merging #666 (d3a1b1c) into master (134c792) will increase coverage by 0.06%. The diff coverage is 100.00%.

:exclamation: Current head d3a1b1c differs from pull request most recent head 2e86b47. Consider uploading reports for the commit 2e86b47 to get more accurate results

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   90.90%   90.97%   +0.06%     
==========================================
  Files          25       24       -1     
  Lines        4356     4177     -179     
==========================================
- Hits         3960     3800     -160     
+ Misses        396      377      -19     

Impacted Files	Coverage Δ
babel/messages/catalog.py	95.65% <100.00%> (-0.07%)	:arrow_down:
babel/messages/frontend.py	87.03% <100.00%> (-0.35%)	:arrow_down:
babel/messages/pofile.py	95.52% <100.00%> (+0.02%)	:arrow_up:

... and 23 files with indirect coverage changes

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[sinoroc]

Anything I can do to get this moving?