Fix ByteBuffer string conversions on JRuby

The getBytes() method this was previously using converts according to
Java's default charset, which means it is potentially not binary-safe.

It has been replaced with a conversion based on a ByteList rather than
an intermediate Java String type, which should sidestep all
encoding-related issues..

ext/nio4r/bytebuffer.c

@@ -265,6 +265,7 @@ static VALUE NIO_ByteBuffer_put(VALUE self, VALUE string)
     struct NIO_ByteBuffer *buffer;
     Data_Get_Struct(self, struct NIO_ByteBuffer, buffer);
 
+    StringValue(string);
     length = RSTRING_LEN(string);
 
     if(length > buffer->limit - buffer->position) {

ext/nio4r/org/nio4r/ByteBuffer.java

@@ -163,10 +163,8 @@ public IRubyObject fetch(ThreadContext context, IRubyObject index) {
 
     @JRubyMethod(name = "<<")
     public IRubyObject put(ThreadContext context, IRubyObject str) {
-        String string = str.asJavaString();
-
         try {
-            this.byteBuffer.put(string.getBytes());
+            this.byteBuffer.put(str.convertToString().getByteList().bytes());
         } catch(BufferOverflowException e) {
             throw ByteBuffer.newOverflowError(context, "buffer is full");
         }

lib/nio/bytebuffer.rb

@@ -111,15 +111,22 @@ def [](index)
 
     # Add a String to the buffer
     #
+    # @param str [#to_str] data to add to the buffer
+    #
+    # @raise [TypeError] given a non-string type
     # @raise [NIO::ByteBuffer::OverflowError] buffer is full
     #
     # @return [self]
-    def <<(str)
+    def put(str)
+      raise TypeError, "expected String, got #{str.class}" unless str.respond_to?(:to_str)
+      str = str.to_str
+
       raise OverflowError, "buffer is full" if str.length > @limit - @position
       @buffer[@position...str.length] = str
       @position += str.length
       self
     end
+    alias << put
 
     # Perform a non-blocking read from the given IO object into the buffer
     # Reads as much data as is immediately available and returns

spec/nio/bytebuffer_spec.rb

@@ -180,6 +180,11 @@
       expect(bytebuffer.limit).to eq capacity
     end
 
+    it "raises TypeError if given a non-String type" do
+      expect { bytebuffer << 42 }.to raise_error(TypeError)
+      expect { bytebuffer << nil }.to raise_error(TypeError)
+    end
+
     it "raises NIO::ByteBuffer::OverflowError if the buffer is full" do
       bytebuffer << "X" * (capacity - 1)
       expect { bytebuffer << "X" }.not_to raise_error