Fix word-extraction substr bug in initialism code

GitHub user prashantv opened a pull request:

https://github.com/apache/thrift/pull/625

Fix word-extraction substr bug in initialism code

Initialisms were not handled in the second word or later. E.g. for the given Thrift file,
```thrift
service S

{ void get_id() }

```

The generated code was using `GetIdArgs` instead of `GetIDArgs`. This was happening because fix_common_initialism was passing the position of the underscore to substr when extracting the word, when substr expects a length.

You can merge this pull request into a Git repository by running:

$ git pull https://github.com/prashantv/thrift initialism_fix

Alternatively you can review and apply these changes as the patch at:

https://github.com/apache/thrift/pull/625.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

This closes #625

---

commit 7df89525116a456b25fff2c903078fece4cb6a2a
Author: Prashant Varanasi <prashant@uber.com>
Date: 2015-09-24T22:06:47Z

Fix word-extraction substr bug in initialism code

Initialisms were not handled in the second word or later.

fix_common_initialism was passing the position of the underscore to
substr when extracting the word, when substr expects a length.

---

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