Respect absolute paths in compute_source_path.

When using compute_source_path to determine the full path of an
asset, if our source begins with '/', we don't want to include
the directory. Examples are illustrative:

> compute_source_path("foo", "stylesheets", "css")
=> "/Users/steve/src/my_app/public/stylesheets/foo.css"
> compute_source_path("/foo", "stylesheets", "css")
=> "/Users/steve/src/my_app/public/foo.css"

Before this patch, the second example would return the same as the
first.

Fixes #5680.

1 files changed, 3 insertions, 0 deletions

diff --git a/actionpack/test/fixtures/public/foo/baz.css b/actionpack/test/fixtures/public/foo/baz.css
new file mode 100644
index 0000000000..b5173fbef2
--- /dev/null
+++ b/actionpack/test/fixtures/public/foo/baz.css
@@ -0,0 +1,3 @@
+body {
+background: #000;
+}