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ARROW-386: [Java] Respect case of struct / map field names

Changes include:
- Remove all toLowerCase() calls on field names in MapWriters.java template file, so that the writers can respect case of the field names.
- Use lower-case keys for internalMap in UnionVector instead of camel-case (e.g. bigInt -> bigint). p.s. I don't know what is the original purpose of using camel case here. It did not conflict because all field names are converted to lower cases in the past.
- Add a simple test case of MapWriter with mixed-case field names.

Author: Jingyuan Wang <jingyuan@live.com>

Closes #261 from alphalfalfa/arrow-386 and squashes the following commits:

cd08145 [Jingyuan Wang] Remove unnecessary handleCase() call
7b28bfc [Jingyuan Wang] Pass caseSensitive Attribute down to nested MapWriters
2fe7bcf [Jingyuan Wang] Separate MapWriters with CaseSensitiveMapWriters
d269e21 [Jingyuan Wang] Configure case sensitivity when constructing ComplexWriterImpl
cba60d1 [Jingyuan Wang] Add option to MapWriters to configure the case sensitivity (defaulted as case-insensitive)
51da2a1 [Jingyuan Wang] Arrow-386: [Java] Respect case of struct / map field names