fix server_default not setting value in defaults, pop server_default fields if set to None, add tests, update docs

docs_src/fields/docs004.py

@@ -0,0 +1,21 @@
+import databases
+import sqlalchemy
+from sqlalchemy import func, text
+
+import ormar
+
+database = databases.Database("sqlite:///test.db")
+metadata = sqlalchemy.MetaData()
+
+
+class Product(ormar.Model):
+    class Meta:
+        tablename = "product"
+        metadata = metadata
+        database = database
+
+    id: ormar.Integer(primary_key=True)
+    name: ormar.String(max_length=100)
+    company: ormar.String(max_length=200, server_default='Acme')
+    sort_order: ormar.Integer(server_default=text("10"))
+    created: ormar.DateTime(server_default=func.now())