Support nested dataclass schema

[NoAnyLove]

Thanks @pilosus for fixing the dataclass-based schema. This is a follow up for #408.

It raises a ValueError excpetion on dataclass-based schema that uses another dataclass-based model. For example,

from pydantic import UrlStr
from pydantic.dataclasses import dataclass

@dataclass
class NavbarButton:
    href: UrlStr

@dataclass
class Navbar:
    button: NavbarButton

print(Navbar.__pydantic_model__.schema())

It raises the following exception,

Traceback (most recent call last):
  File "demo_dataclass.py", line 14, in <module>
    print(Navbar.__pydantic_model__.schema())
  File "E:\Python36\lib\site-packages\pydantic\main.py", line 406, in schema
    s = model_schema(cls, by_alias=by_alias)
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 206, in model_schema
    model, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 490, in model_process_schema
    model, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 516, in model_type_schema
    f, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 258, in field_schema
    ref_prefix=ref_prefix,
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 465, in field_type_schema
    ref_prefix=ref_prefix,
  File "E:\Python36\lib\site-packages\pydantic\schema.py", line 685, in field_singleton_schema
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: button type=NavbarButton required

[NoAnyLove]

I tried to make a fix for it. The code change is in #475. Nott quite sure if the fix is elegant or not.