Add Transloadit-Client header

[Acconut]

All SDKs should include the Transloadit-Client header for every Transloadit-related request they are sending (tus excluded). This is helpful for our support, so we can easily identify which SDKs in which version they are using.

For example, here is how the Java SDK does it: https://github.com/transloadit/java-sdk/blob/4a631daa808e4ebf58ea9e30855cde1c5ddd555c/src/main/java/com/transloadit/sdk/Request.java#L74

The complete header with value should look like this: https://github.com/transloadit/java-sdk/blob/4a631daa808e4ebf58ea9e30855cde1c5ddd555c/src/test/java/com/transloadit/sdk/RequestTest.java#L68

[kvz]

Refs https://github.com/transloadit/team-internals/issues/26