Reproduction steps:

1. Open "MinimalRepro.zip" project
2. Open Fiddler 4
3. Enable Filters
4. In the field "Show only if URL contains" write "gamespark"
5. Go to the "Inspector" window
6. In Unity, enter Play mode
7. Click on the first button
8. In Fiddler, click on the field that has a host of "config2.gamesparks.net" (Indicated by red text)
9. In the top right side window click on "Raw"
10. Notice the boundary value on line 6
11. Notice that the starting boundary is on line 12
12. Notice that at the end of the file, there is no ending boundary

Expected Result: Unity WebRequest's MultipartFormFileSection has a closing boundary
Actual Result: Unity WebRequest's MultipartFormFileSection doesn't have a closing boundary

Reproduced with: 2018.2.0b3, 2018.1.0f2, 2017.4.2f2, 2017.3.2f1, 2017.2.3f1, 2017.1.3p4, 2017.1.0a1