I've added a pull request hook to one of my private repos.
I can't see the content type application/json being set in your request + I believe this to be the cause of a few problems I'm having.

The headers which are being set are:

host:'',
content-length': '395',
accept-encoding: 'gzip, deflate',
authorization: '',
user-agent: 'Bitbucket.org',
x-newrelic-id: '',
x-newrelic-transaction: ''

I've been referring to the docs on this page:
https://confluence.atlassian.com/display/BITBUCKET/Pull+Request+POST+hook+management

Which state:
"The content header of the POST has an application/json type"

Any ideas?
thanks