Ensure Vec<T> can be used in #[changeset_for]

Usually we implement all the various `AsExpression` for refs in the
`expression_impls!` macro. However, that macro doesn't work with generics,
and we forgot to handle `Vec<T>` (I think I figured the slice impl was
enough for some reason).

I tried to write a test for this change, but there are actually several
other bugs that prevent me from testing this properly. See #65 & #66.

Fixes #63.

CHANGELOG.md

@@ -12,6 +12,11 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
 * Add print_sql! and debug_sql! macros to print out and return sql strings from
   QueryFragments.
 
+### Fixed
+
+* `#[changeset_for]` can now be used with structs containing a `Vec`. Fixes
+  [#63](https://github.com/sgrif/diesel/issues/63).
+
 ## [0.3.0] 2015-12-04
 
 ### Changed

diesel/src/types/impls/array.rs

@@ -90,6 +90,17 @@ impl<ST, T> AsExpression<Array<ST>> for Vec<T> where
     }
 }
 
+impl<'a, ST, T> AsExpression<Array<ST>> for &'a Vec<T> where
+    ST: NativeSqlType,
+    T: ToSql<ST>,
+{
+    type Expression = Bound<Array<ST>, Self>;
+
+    fn as_expression(self) -> Self::Expression {
+        Bound::new(self)
+    }
+}
+
 impl<'a, ST, T> ToSql<Array<ST>> for &'a [T] where
     ST: NativeSqlType,
     T: ToSql<ST>,