mem: return error when reading a directory (fixes #430)

[nghiack7]

Summary

afero.ReadFile(fs, dirPath) on a MemMapFs silently returned empty content with a nil error. The real OS filesystem returns an error (read /some/dir: is a directory). This inconsistency was reported in #430.

Root cause

(*File).Read delegates to ReadAt, which had no guard for directory-backed files. It would simply return n=0, err=nil because the file data slice is empty for directories.

Fix

Added a directory check at the top of ReadAt:

if f.fileData.dir {
    return 0, &os.PathError{Op: "read", Path: f.fileData.name, Err: errors.New("is a directory")}
}

Read already delegates to ReadAt, so both are covered by one guard.

Test

Added TestReadFileOnDirectory in util_test.go that:

1. Creates a MemMapFs directory
2. Calls afero.ReadFile on it
3. Asserts a non-nil error is returned

$ go test ./... -run TestReadFile
ok      github.com/spf13/afero  0.6s

Full test suite passes without regressions.

Fixes #430

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[nghiack7]

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