Don't null size field when scale specified

This is an issue that I come upon using propel migration. It always tried to migrate my decimal column with the same values.

After inspecting closely I have figured that reverse schema parser sets null to $size variable (even if $scale) is specified.

This way, when migrator compares columns it has $size = 10 in schema.xml and $size = null in database (because reverse mysql class changes native sizes to null).

Real life example:
XML:
latitude: { phpName: Latitude, type: DECIMAL, size: 10, scale: 7, required: false }

DB:
  `latitude` decimal(10,7) DEFAULT NULL,


produces migration queries as such:

getUpSQL:
ALTER TABLE `venues` CHANGE `latitude` `latitude` DECIMAL(10,7);

getDownSQL:
ALTER TABLE `venues` CHANGE `latitude` `latitude` DECIMAL;


even though database is already set propely with size and scale.
Update generator/lib/reverse/mysql/MysqlSchemaParser.php

fix typo
add tests for issue #621
Merge pull request #621 from fliespl/patch-1
Don't null size field when scale specified