fix accept header

[systemfreund]

The current version causes the following exception when using openjdk 1.8.0_66:

IllegalArgumentException: Illegal character(s) in message header field: Accepts:

Stacktrace (most recent call first):

    at sun.net.www.protocol.http.HttpURLConnection.checkMessageHeader
    at sun.net.www.protocol.http.HttpURLConnection.isExternalMessageHeaderAllowed
    at sun.net.www.protocol.http.HttpURLConnection.setRequestProperty
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.setRequestProperty
    at com.messagebird.MessageBirdServiceImpl.getConnection
    at com.messagebird.MessageBirdServiceImpl.getJsonData
    at com.messagebird.MessageBirdServiceImpl.sendPayLoad
    at com.messagebird.MessageBirdClient.sendMessage

openjdk version "1.8.0_66-internal"
OpenJDK Runtime Environment (build 1.8.0_66-internal-b17)
OpenJDK 64-Bit Server VM (build 25.66-b17, mixed mode)

[samwierema]

Thanks for the pull request! All seems to look good. We'll merge this and deploy a new version to Maven Central today.