[#1776] Fix bug when upserting [] on JSON column in the datastore

We were testing that the new value wasn't `None` using `if value`. As `[]` is a
falsy value, we never went through that `if` and ended up trying to run an
invalid SQL query.

This commit fixes that by explictly testing `if value is not None`.

This was first discovered in
http://stackoverflow.com/questions/24207065/inserting-empty-arrays-in-json-type-fields-in-datastore
[#1819] Upgrade Requests to 2.3.x

Only a minor change in the resource proxy extension (look like the
content-length header is no longer added by default).
Role translations.
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Add translation of lines from 4400 to 4800
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Preklad riadkov 600-800
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preklad riadkov 0 - 600
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Fix translation
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Fix some translation
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translated lines 801-1600
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Translation for rows 4002-4401
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Dočasná verzia
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Slovak translation lines 4000-4400 and 1 correction in Czech translation
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