OFXClient.http_headers - correct typo "Content-type" -> "Content-Type"

Author: csingley

ofxtools/Client.py

@@ -317,7 +317,7 @@ def http_headers(self) -> Dict[str, str]:
         # which apparently displeases some FIs
         return {
             "User-Agent": self.useragent,
-            "Content-type": mimetype,
+            "Content-Type": mimetype,
             # Apparently Amex is unhappy unless it sees a MIME type of application/xml
             # with some quality rating - ANY quality rating, it seems.
             "Accept": "*/*, {}, application/xml;q=0.9".format(mimetype),