Merge pull request #621 from vapor/querybuilder-copy

Add support for copying query builders.

Sources/Fluent/QueryBuilder/QueryBuilder+Copy.swift

@@ -0,0 +1,16 @@
+extension QueryBuilder {
+    // MARK: Copy
+
+    /// Creates a copy of the current query builder. Useful for constructing two similar, but not identical, queries.
+    ///
+    /// Example: two queries sorted by different criteria.
+    ///
+    ///     let baseQuery = try User.query(on: conn).filter(\.name == "foo")
+    ///     let sortedByID = baseQuery.copy().sort(\.id, .ascending)
+    ///     let sortedByName = baseQuery.copy().sort(\.name, .ascending)
+    ///
+    /// - returns: A copy of the current query builder that can be manipulated independently of the original.
+    public func copy() -> Self {
+        return .init(query: self.query, on: self.connection, resultTransformer: self.resultTransformer)
+    }
+}