`nimble publish` works through all the steps except for the final PR creation
Hello,

Today I was trying to publish a new package (now PR'ed manually: https://github.com/nim-lang/packages/pull/1076).

I tried the `nimble publish` command and I got:

```
> nimble publish
      Info: Using GitHub API Token in file: /home/kmodi/.nimble/github_api_token
   Success: Verified as kaushalmodi
    Copying packages fork into: /tmp/nimble-packages-fork
   Updating the fork
    Prompt: Whitespace separated list of tags?
    Answer: dpi-c systemverilog foreign-function interface
    Pushing to remote of fork.
       Info Creating PR
../../../lib/pure/httpclient.nim(997) responseContent
Error: unhandled exception: 422 Unprocessable Entity [HttpRequestError]
```

So it successfully created the fork, a branch with the correct changes in that fork, but just failed at the PR creation.

---

## Version info

### `nimble --version`

```
nimble v0.9.0 compiled at 2019-05-06 21:46:20
git hash: f17eaef7957da23aab6e375400b37227394a632a
```

### `nim --version`

```
Nim Compiler Version 0.19.9 [Linux: amd64]
Compiled at 2019-05-06
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 2475d92c36e26e44372175c2cf4c21fbab084619
active boot switches: -d:release
```