ArgumentException: Cannot set Request Header Content-Length - name contains illegal characters or is not user-overridable

How to reproduce:
1. Open user attached project
2. Enter playmode

Expected behaviour: Successful attempt to send files to Firebase (404 since we don't have access)
Actual behaviour: Argument exception

Regression introduced in: 2017.1.0b1
Reproduced on: 2017.1.0b1, 2017.1.0b5, 2017.1.0b8, 2017.2.0a2
Not reproduced on: 5.5.4f1, 5.6.2p2

Full error from 2017.1.0b1:
ArgumentException: Cannot set Request Header Content-Length - name contains illegal characters or is not user-overridable
UnityEngine.Networking.UnityWebRequest.SetRequestHeader (System.String name, System.String value) (at C:/buildslave/unity/build/artifacts/generated/common/modules/UnityWebRequest/WebRequestBindings.gen.cs:473)
UnityEngine.WWW..ctor (System.String url, System.Byte[] postData, System.Collections.Generic.Dictionary`2 headers) (at C:/buildslave/unity/build/Runtime/WebRequestWWW/UWRWWW.cs:60)
Firebase.Unity.WWWHttpRequest+<SendUnityRequest>c__Iterator0.MoveNext ()
UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at C:/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)
UnityEngine.MonoBehaviour:StartCoroutine(IEnumerator)
Firebase.Unity.UnitySynchronizationContext:<PostCoroutine>m__0(Object)
Firebase.Unity.<Start>c__Iterator0:MoveNext()
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)