Adds support GraphML support for long

[jamesdbaker]

Fixes #1140 by adding GraphML support for "long", along with corresponding unit tests

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[IvanIsCoding]

Thanks so much for finding the bug AND contributing the fix! Overall the code seems ready and the unit test is appropriate.

I think the only missing item is the CLA signing and then we should be good to go

[coveralls]

Pull Request Test Coverage Report for Build 8298916674

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.48%

---

Totals
Change from base Build 8210134916:	0.0%
Covered Lines:	17160
Relevant Lines:	17786

---

💛 - Coveralls