Fix end-of-headers marker detection

The current heuristic detection isn't RFC compliant.   It broke with the following (valid but stupid) header:

```
References: <7FD4C8B39D0B18439D432908FA1385D302628A82@a16664.marins.lan>
                                                                              
                                                        <A7D88A1E18E690468AB90A60B0E8DAB1787F2C9C@a16664.marins.lan>
 <A7D88A1E18E690468AB90A60B0E8DAB1AB7F0801@a16664.marins.lan>
                              <7FD4C8B39D0B18439D432908FA1385D30325EB79@a16664.marins.lan>
 <1A062DA335F9AF45A55C9AD640966BBF0117830D0040@HLBEHT11>
```

The 2nd line of the header has a bunch of spaces and a \r\n at the end - it should be treated as a header continuation but was being detected as the EOH marker instead.