Add support for custom request headers

Similar to https://github.com/apache/tinkerpop/commit/29e61312905301cb8f4f32469ab712821705ecf5

added 1 commit

• fca384af - Adpating to changes in gremlinpython-3.4.4

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@grzn Hi I am going through now and about to do a version release on Goblin. Will be reviewing this shortly and try to get it in this release.

Thanks for your contribution.