Don't swallow Error message/stack when using formatter

Fixes winstonjs#1178

When we use a formatter and pass an `Error` object in as `meta`,
we end up removing the Error's message and stack information when
`decycle`ing it. `clone` checks if the passed-in object is an `Error`
and does not `decycle` it in that case, but when we use a formatter,
this check does not happen because we `clone` the whole `options` object
instead of just cloning `options.meta`.

Simply removing the `&& !(options.meta instanceof Error)` from this line
fixes the problem, by forcing it to `clone` `meta` the first time, when
the `instanceof Error` check will catch it and copy `message` and `stack`
into a new, non-Error object. So when that's buried in `options.meta`,
re-`clone`ing it won't hurt.

Reading the code it seems to not have any bad side-effects. In the places
that use meta, double-`clone`ing is done anyway for other non-`Error`
objects, and the if conditions (`typeof meta !== 'object' && meta != null`)
would be false either way for `Error` objects or non-`Error` objects.

lib/winston/common.js

@@ -143,7 +143,7 @@ exports.log = function (options) {
         : exports.timestamp,
       timestamp   = options.timestamp ? timestampFn() : null,
       showLevel   = options.showLevel === undefined ? true : options.showLevel,
-      meta        = options.meta !== null && options.meta !== undefined && !(options.meta instanceof Error)
+      meta        = options.meta !== null && options.meta !== undefined
         ? exports.clone(options.meta)
         : options.meta || null,
       output;

test/custom-formatter-test.js

@@ -28,7 +28,7 @@ function assertFileFormatter (basename, options) {
       // We must wait until transport file has emitted the 'flush'
       // event to be sure the file has been created and written
       transport.once('flush', this.callback.bind(this, null, filename));
-      transport.log('info', 'What does the fox say?', null, function () {});
+      transport.log('info', 'What does the fox say?', new Error("foo"), function () {});
     },
     "should log with the appropriate format": function (_, filename) {
       var data = fs.readFileSync(filename, 'utf8');
@@ -58,7 +58,7 @@ vows.describe('winston/transport/formatter').addBatch({
         formatter: {}
       }),
       "and function value with custom format": assertFileFormatter('customFormatter', {
-        pattern: /^\d{13,} INFO What does the fox say\?/,
+        pattern: /^\d{13,} INFO What does the fox say\?.*foo/,
         json: false,
         timestamp: function() {
           return Date.now();