vmime: prevent loss of a space during text::createFromString (#306)

```
mailbox(text("Test München West", charsets::UTF_8), "[email protected]").generate();
```

produces

```
=?us-ascii?Q?Test_?= =?utf-8?Q?M=C3=BCnchen?= =?us-ascii?Q?West?= <[email protected]>
```

The first space between ``Test`` and ``München`` is encoded as an
underscore along with the first word: ``Test_``. The second space
between ``München`` and ``West`` is encoded with neither of the two
words and thus lost. Decoding the text results in ``Test
MünchenWest`` instead of ``Test München West``.

This is caused by how ``vmime::text::createFromString()`` handles
transitions between 7-bit and 8-bit words: If an 8-bit word follows a
7-bit word, a space is appended to the previous word. The opposite
case of a 7-bit word following an 8-bit word *misses* this behaviour.

When one fixes this problem, a follow-up issue appears:

``text::createFromString("a b\xFFc d")`` tokenizes the input into
``m_words={word("a "), word("b\xFFc ", utf8), word("d")}``. This
"right-side alignment" nature of the whitespace is a problem for
word::generate():

As per RFC 2047, spaces between adjacent encoded words are just
separators but not meant to be displayed. A space between an encoded
word and a regular ASCII text is not just a separator but also meant
to be displayed.

When word::generate() outputs the b-word, it would have to strip one
space, but only when there is a transition from encoded-word to
unencoded word. word::generate() does not know whether d will be
encoded or unencoded.

The idea now is that we could change the tokenization of
``text::createFromString`` such that whitespace is at the *start* of
words rather than at the end. With that, word::generate() need not
know anything about the next word, but rather only the *previous*
one.

Thus, in this patch,

1. The tokenization of ``text::createFromString`` is changed to
   left-align spaces and the function is fixed to account for
   the missing space on transition.
2. ``word::generate`` learns how to steal a space character.
3. Testcases are adjusted to account for the shifted
   position of the space.

Fixes: #283, #284

Co-authored-by: Vincent Richard <[email protected]>

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