[MS Edge bug] Response header string starts with \n

Created by: davidgarry

Trim headerStr to fix a MS Edge bug with xhr.getAllResponseHeaders method which send a string starting with a "\n" character

Sources :

• https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/5403401/
• https://connect.microsoft.com/IE/feedbackdetail/view/1929694/ie-edge-incorrectly-return-the-header-with-a-leading-new-line-character
• https://github.com/exceptionless/Exceptionless.JavaScript/issues/23