mutable-type should imply mutable in tahoe put

[kevan]

Currently, if you do tahoe put --mutable-type=mdmf some_file, an immutable (CHK or LIT) file is created, since tahoe put only pays attention to the --mutable-type option if the --mutable option is also given. This is surprising; we should treat --mutable-type as implying --mutable. Then --mutable can be understood to mean "upload a mutable file using whatever the default mutable type is", and --mutable-type can be understood as a way to override that default.

(This improvement was suggested by davidsarah in ​#393, comment 141.)