time: sql_type for DateTimeProxy without backend

With the time feature enabled and no active database backend
compilation fails because no sql_type is specified for DateTimeProxy.

This is fixed by always specifying SQL type Datetime regardless of the
enabled backends.

diesel/src/type_impls/date_and_time.rs

@@ -57,7 +57,7 @@ mod time {
 
     use crate::deserialize::FromSqlRow;
     use crate::expression::AsExpression;
-    use crate::sql_types::{Date, Time, Timestamp};
+    use crate::sql_types::{Date, Datetime, Time, Timestamp};
 
     #[derive(AsExpression, FromSqlRow)]
     #[diesel(foreign_derive)]
@@ -81,11 +81,11 @@ mod time {
 
     #[derive(AsExpression, FromSqlRow)]
     #[diesel(foreign_derive)]
+    #[diesel(sql_type = Datetime)]
     #[cfg_attr(
         feature = "postgres_backend",
         diesel(sql_type = crate::sql_types::Timestamptz)
     )]
     #[cfg_attr(feature = "sqlite", diesel(sql_type = crate::sql_types::TimestamptzSqlite))]
-    #[cfg_attr(feature = "mysql_backend", diesel(sql_type = crate::sql_types::Datetime))]
     struct DateTimeProxy(OffsetDateTime);
 }