reading from assets.Files not concurrency safe, or repeated use safe

[rbtcollins]

I followed the gin example code that used go-asset-builder, which looks roughly like the below; I then had mysterious test failures with empty result bodies until I put in the panic that is in the below code (and not in the example I cribbed from :)).

The bug is fairly straight forward - the Files instance is already 'open' so the buffers are never recreated and everything can only be read once. It also humorously (or not) means its not concurrency safe - multithreaded reads can interleave different parts of the file and move the buffer pointer forward.

	t := template.New("")
	for name, file := range assets.Files {
		if file.IsDir() || !strings.HasSuffix(name, ".gohtml") {
			continue
		}
		h, err := ioutil.ReadAll(file)
		if err != nil {
			return nil, err
		}
		if len(h) == 0 {
			panic("zero length template")
		}
		t, err = t.New(name).Parse(string(h))
		if err != nil {
			return nil, err
		}
	}

With that assert, my tests rather than failing with empty bodies fail when instantiating my router:

--- FAIL: TestServerTestSuite (0.00s)
    --- FAIL: TestServerTestSuite/TestRootDocHTML (0.00s)
        .../suite.go:61: test panicked: zero length template
    --- FAIL: TestServerTestSuite/TestRootDocJSON (0.00s)
        .../suite.go:61: test panicked: zero length template