ARROW-2503: [Python] Prevent trailing space character for string statistics

[jneuff]

The trailing space is added in parquet-cpp.pyarrow calls the function FormatStatValue which adds the trailing space (https://github.com/apache/parquet-cpp/blob/master/src/parquet/types.cc#L52).

https://issues.apache.org/jira/browse/PARQUET-1283 is about fixing this behavior. Once the corresponding PR is merged into parquet-cpp and pyarrow is synced, the test_parquet.py:test_parquet_column_statistics_api will break for str. This PR fixes that breakage.

[xhochy]

I guess this depends on the PR in parquet-cpp as the statistics tests are failing?

[jneuff]

Sorry, I was a bit to eager to open this PR. I want to solve it without depending on the PR in parquet-cpp.

I'll reiterate.

[jneuff]

I updated the scope of this PR, see the description above.

[pitrou]

@jneuff, could you rebase this PR so that we get a fresh CI build?

[pitrou]

I opened a competing PR #1973 to get CI as soon as possible. Let's see which one ends first.

[xhochy]

@pitrou you should also have the right to push to contributor branches on github, no need to open new PRs if it's only a rebase.

[pitrou]

In the past I've messed up things while trying to push to contributor branches :-)

[pitrou]

Also the goal is to get an AppVeyor build on my account.

[pitrou]

AppVeyor build succeeded at https://ci.appveyor.com/project/pitrou/arrow/build/1.0.359