Expose _becomeMimePart() to allow signed message to be implemented correctly
Enable signed message to handle body text/plain part to be set via setBody();

+ Corrected bug in test, the content-type MUST be text/plain
Dropping unused file, was meant for a "internal extension" never released
Implementing better Openssl error handling
merged branch xdecock/SignFix (PR #283)

This PR was merged into the master branch.

Commits
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9bc3134 Implementing better Openssl error handling
38d1ef1 Dropping unused file, was meant for a "internal extension" never released
5938da3 Enable signed message to handle body text/plain part to be set via setBody();
bcf4ee4 Expose _becomeMimePart() to allow signed message to be implemented correctly

Discussion
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Sign fix

Same as previous pull request, but with better segmented commits, with an up-to-date tree, and not messing with myself.

Basically,

+ Removed an old file that never worked
+ Allowed signing plugins to work on messages plain/text alternative injected via ->setBody method();
+ Corrected a bug in an SMime Test.