Set 'utf8mb4' when exporting without '--default-character-set' option

[ryotsun]

Fixes: #144

I'm not sure whether this approach is correct.
Kindly check this PR in your time.

[wojsmol]

@schlessera What if the database is in utf8 and by setting utf8mb4 as default same backup script will be broken?

[schlessera]

@wojsmol Good question. As far as I know, utf8mb4 is fully backwards compatible with utf8, so I assume any utf8 export should just work to be imported into utf8mb4. Any potential data loss would already have happened before the export, as utf8 would have already stripped any 4-byte code points.

But I'm not an expert in DB encodings. Am I maybe missing something here?

[wojsmol]

Regarding characters maybe, but for shore there are deferences in length of the indexes - see this make/core blog post linked in the original issue.

[schlessera]

@wojsmol I now added a detection of the actual character set of the wp_posts table. I think this should be safe enough now to use as a default, and reduce the risk of people actually losing their emojis on migration.

What do you think about that approach?