BUG: pandas.read_sql missing table schema input

[vinodMS]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd

pd.read_sql('test_data', 'postgres:///db_name')

Issue Description

This specific method wraps around the read_sql_query and read_sql_table methods, where read_sql checks if the sql parameter contains a valid table name, if not it assumes that the sql parameter contains a sql query and routes the request to read_sql_query.

I'm using a SQL Database, hence read_sql uses SQLDatabase.has_table method to determine whether or not a valid table name was provided as part of the sql argument.

The problem here is that the following line in the has_table method requires a table schema name(if default schema table is not used)
return insp.has_table(name, schema or self.meta.schema)
but there is no schema name passed when instantiating the SQLDatabase class nor is a schema name given when calling the has_table method from read_sql.

Expected Behavior

df = pd.read_sql('test_data', 'postgres:///db_name') should be able to find the 'test_data' table that has a custom table schema.

in order to solve the bug the following could be done,

1. Add a schema parameter to the read_sql method (similar to that of read_sql_table)
2. And then this line should be updated to this - _is_table_name = pandas_sql.has_table(sql, schema)

Happy to create a PR if agreed.

Installed Versions

INSTALLED VERSIONS

commit : None
python : 3.8.3.final.0
python-bits : 64
OS : Darwin
OS-release : 20.5.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.0.5
numpy : 1.18.5
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 49.2.0.post20200714
Cython : 0.29.21
pytest : 5.4.3
hypothesis : None
sphinx : 3.1.2
blosc : None
feather : None
xlsxwriter : 1.2.9
lxml.etree : 4.5.2
html5lib : 1.1
pymysql : 1.0.2
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.16.1
pandas_datareader: None
bs4 : 4.9.1
bottleneck : 1.3.2
fastparquet : None
gcsfs : None
lxml.etree : 4.5.2
matplotlib : 3.2.2
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.4
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.4.3
pyxlsb : None
s3fs : None
scipy : 1.5.0
sqlalchemy : 1.3.18
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.9
numba : 0.50.1