add some missing methods/trait bounds (#318)

This PR:
* adds a missing ?Sized bound on Message::is_answer,
* adds MessageBuilder::as_target,
* impls FreezeBuilder for StaticCompressor.

src/base/message.rs

@@ -422,7 +422,10 @@ impl<Octs: Octets + ?Sized> Message<Octs> {
     /// The method checks whether the ID fields of the headers are the same,
     /// whether the QR flag is set in this message, and whether the questions
     /// are the same.
-    pub fn is_answer<Other: Octets>(&self, query: &Message<Other>) -> bool {
+    pub fn is_answer<Other: Octets + ?Sized>(
+        &self,
+        query: &Message<Other>,
+    ) -> bool {
         if !self.header().qr()
             || self.header().id() != query.header().id()
             || self.header_counts().qdcount()

src/base/message_builder.rs

@@ -1706,6 +1706,11 @@ impl<'a, Target: Composer + ?Sized> OptBuilder<'a, Target> {
         let start = self.start;
         OptHeader::for_record_slice_mut(&mut self.target.as_mut()[start..])
     }
+
+    /// Returns a reference to the underlying octets builder.
+    pub fn as_target(&self) -> &Target {
+        self.target
+    }
 }
 
 //------------ StreamTarget --------------------------------------------------
@@ -2050,6 +2055,14 @@ impl<Target: Truncate> Truncate for StaticCompressor<Target> {
     }
 }
 
+impl<Target: FreezeBuilder> FreezeBuilder for StaticCompressor<Target> {
+    type Octets = Target::Octets;
+
+    fn freeze(self) -> Self::Octets {
+        self.target.freeze()
+    }
+}
+
 //------------ TreeCompressor ------------------------------------------------
 
 /// A domain name compressor that uses a tree.