Add header folding to MimeHeaderEncoder

[cketti]

This fixes a crash reported via Google Play (for K-9 Mail 6.800).

Exception java.lang.RuntimeException: An error occurred while executing doInBackground()
  at android.os.AsyncTask$4.done (AsyncTask.java:415)
  at java.util.concurrent.FutureTask.finishCompletion (FutureTask.java:381)
  at java.util.concurrent.FutureTask.setException (FutureTask.java:250)
  at java.util.concurrent.FutureTask.run (FutureTask.java:269)
  at android.os.AsyncTask$SerialExecutor$1.run (AsyncTask.java:305)
  at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:644)
  at java.lang.Thread.run (Thread.java:1012)
Caused by java.lang.AssertionError: Invalid header
  at com.fsck.k9.mail.internet.MimeHeader.checkHeader (MimeHeader.kt:109)
  at com.fsck.k9.mail.internet.MimeHeader.requireValidHeader (MimeHeader.kt:90)
  at com.fsck.k9.mail.internet.MimeHeader.addHeader (MimeHeader.kt:31)
  at com.fsck.k9.mail.internet.MimeHeader.setHeader (MimeHeader.kt:44)
  at com.fsck.k9.mail.internet.MimeMessage.setHeader (MimeMessage.java:433)
  at com.fsck.k9.mail.internet.MimeMessage.setSubject (MimeMessage.java:281)
  at com.fsck.k9.message.MessageBuilder.buildHeader (MessageBuilder.java:106)
  at com.fsck.k9.message.MessageBuilder.build (MessageBuilder.java:92)
  at com.fsck.k9.message.SimpleMessageBuilder.buildMessageInternal (SimpleMessageBuilder.java:33)
  at com.fsck.k9.message.MessageBuilder$1.doInBackground (MessageBuilder.java:532)
  at com.fsck.k9.message.MessageBuilder$1.doInBackground (MessageBuilder.java:529)
  at android.os.AsyncTask$3.call (AsyncTask.java:394)
  at java.util.concurrent.FutureTask.run (FutureTask.java:264)
Caused by com.fsck.k9.mail.internet.MimeHeaderParserException:
  at com.fsck.k9.mail.internet.UnstructuredHeaderChecker.checkHeaderValue (MimeHeaderChecker.kt:65)
  at com.fsck.k9.mail.internet.MimeHeaderChecker.checkHeader (MimeHeaderChecker.kt:25)
  at com.fsck.k9.mail.internet.MimeHeader.checkHeader (MimeHeader.kt:106)

The problem was that long lines were encoded using the encoded word encoding, but the result was not folded. So the maximum line length (998 characters) could be exceeded.

To avoid sending invalid messages that e.g. exceed the maximum line length we use MimeHeaderChecker to catch those errors and crash the app.

[wmontwe]

Added some readability and test findings.

[wmontwe]

👍