add default coerce to any Select2Field

[samialfattani]

run-time error is raised if Select2Field did'nt include coerce=int in form_args

Here is how to replicate the error

  class Model2(db.Model):  
      def __init__(self,int_field=None):
          self.int_field = int_field

      int_field = Column(Integer)

  class MyModelView(ModelView):
      form_columns = ["int_field"]
      form_choices = {"int_field": [("101", "101"), ("150", "150")]}
      form_args = {
          "int_field": {"validators": [NumberRange(min=100, max=199)]},
      }

This PR adds a default coerce function if it is not provided explicitly based on the Column's type

the added tests won't pass without changes in this PR

[ElLorans]

Why do you need to change typing? Do we need to change all other orm libraries?

[samialfattani]

Why do you need to change typing? Do we need to change all other orm libraries?

what do you mean? i'm not changing the typing. This PR just extract the right coerce function that match the Column's type.

[ElLorans]

Sorry, let me be more specific. Why not editing T_FIELD_ARGS_VALIDATORS_ALLOW_BLANK

[samialfattani]

Sorry, let me be more specific. Why not editing T_FIELD_ARGS_VALIDATORS_ALLOW_BLANK

the T_FIELD_ARGS_VALIDATORS_ALLOW_BLANK is being used in other 2 functions _get_description and _convert_relation and i don't want those funcitons to be harmed if just edit it. but any i can have a try if you say so.

[samialfattani]

it is ready for review now