Fixes #182. Negative numbers serialized with quotes.

Author: dpbevin

yaml/src/main/java/com/fasterxml/jackson/dataformat/yaml/YAMLGenerator.java

@@ -172,7 +172,7 @@ private Feature(boolean defaultState) {
 
     protected final static long MIN_INT_AS_LONG = (long) Integer.MIN_VALUE;
     protected final static long MAX_INT_AS_LONG = (long) Integer.MAX_VALUE;
-    protected final static Pattern PLAIN_NUMBER_P = Pattern.compile("[0-9]*(\\.[0-9]*)?");
+    protected final static Pattern PLAIN_NUMBER_P = Pattern.compile("-?[0-9]*(\\.[0-9]*)?");
     protected final static String TAG_BINARY = Tag.BINARY.toString();
 
     // for field names, leave out quotes

yaml/src/test/java/com/fasterxml/jackson/dataformat/yaml/ser/GeneratorWithMinimizeTest.java

@@ -186,6 +186,13 @@ public void testQuoteNumberStoredAsString() throws Exception
 
         assertEquals("---\n" +
                 "key: 2.0.1.2.3", yaml);
+
+        content.clear();
+        content.put("key", "-60");
+        yaml = mapper.writeValueAsString(content).trim();
+
+        assertEquals("---\n" +
+                "key: \"-60\"", yaml);
     }
 
     public void testNonQuoteNumberStoredAsString() throws Exception