Check whether column is numeric along with unsigned check (#163)

Attempt to fix #159. It seems there are cases when `API.isunsigned` will
return true for a timestamp column; this is probably due to some kind of
database or column setting when the table was created, but nonetheless,
it causes issues because if `API.isunsigned`, then we try to call
`unsigned(T)`, which isn't defined for `DateTime`. As opposed to
pirating that definition in MySQL.jl, here we attempt to also check the
`NUM_FLAG`, which should tell us whether the field is numeric or not and
hopefully avoid ever seeing `API.isunsigned` for timestamp columns.

src/api/apitypes.jl

@@ -89,7 +89,7 @@ struct MYSQL_FIELD
     extension::Ptr{Cvoid}
 end
 notnullable(field) = (field.flags & NOT_NULL_FLAG) > 0
-isunsigned(field) = (field.flags & UNSIGNED_FLAG) > 0
+isunsigned(field) = (field.flags & NUM_FLAG) > 0 && (field.flags & UNSIGNED_FLAG) > 0
 isbinary(field) = (field.flags & BINARY_FLAG) > 0
 
 const MYSQL_FIELD_OFFSET = Cuint

src/api/consts.jl

@@ -206,6 +206,7 @@ const MYSQL_TIMESTAMP_TIME     = 2
 const NOT_NULL_FLAG = UInt32(1)
 const UNSIGNED_FLAG = UInt32(32)
 const BINARY_FLAG = UInt32(128)
+const NUM_FLAG = UInt32(32768)
 const MYSQL_NO_DATA = 100
 
 const MYSQL_DEFAULT_PORT = 3306