Fix-xDbType

[siggi-k]

x-db-type will no longer be reformatted from now on.
x-db-type is now the final entry in db.

nullalble property is handled correctly.

usecase

created_at:
  readOnly: true
  type: string
  format: datetime
  x-db-type: datetime
  nullable: false

fixes #100
fixes #99

[cebe]

Tests are failing after these changes.

[cebe]

condition looks hard to read, should be positive check instead of negative.

[SOHELAHMED7]

Condition:

	is mysql	is mariadb	type in array	final	global negation
mysql json	T	!F=T	T	T	F
mysql nonjson	T	!F=T	F	F	T
mariadb json	F	!T=F	T	F	T
mariadb nonjson	F	!T=F	F	F	T

---

Default are not allowed for BLOB, TEXT, GEOMETRY, and JSON in MySQL prior to MySQL 8.0.13

Curious to know why it is allowed for MariaDB?

[cebe]

mariadb allows default value for JSON column at least, have been using that a few times. e.g. data JSON NOT NULL DEFAULT '{}'

[SOHELAHMED7]

Completing this PR in #112