mem: ReadFile returns an error when filename is a directory

[amitmishra11]

Fixes #201

Bug

afero.ReadFile on a MemMapFs silently succeeds with empty content when the path is a directory, instead of returning an error like the real os.ReadFile does (EISDIR).

fs := afero.NewMemMapFs()
fs.MkdirAll("/somedir", 0755)
data, err := afero.ReadFile(fs, "/somedir")
// data == "", err == nil  (expected: err != nil)

Root cause

An in-memory directory's FileData.data buffer is empty, the same as an empty regular file. mem.File.ReadAt had no check for fileData.dir, so reading a directory returned (0, io.EOF) exactly like reading an empty file would. File.Read and afero.ReadFile's underlying readAll treat a first io.EOF as a normal empty read (not an error), so the whole call chain returns ("", nil) with no indication anything was wrong.

Readdir already has the equivalent inverse check (if !f.fileData.dir { return error }), so this brings ReadAt in line with that existing convention.

Fix

Added a directory check at the top of mem.File.ReadAt that returns &os.PathError{Op: "read", Path: ..., Err: syscall.EISDIR}, matching the real os.File's behavior when Read is called on a directory.

Scoped to the read path only (matching the issue title/report) — did not touch Write/WriteAt, which is a separate question not raised by this issue.

Tests

Added TestReadFileOnDirectory in ioutil_test.go, following the existing TestReadFile pattern: creates a directory via MkdirAll, calls Afero.ReadFile on it, and asserts an error is returned.

I verified the test actually catches the regression: temporarily removed the new directory check in ReadAt and reran the test — it failed with ReadFile somedir: error expected, got nil (contents=""), reproducing the exact symptom from the issue. Restoring the fix makes it pass again.

Verification

• go build ./... — clean
• go vet ./... — clean
• go test ./... — all pass except TestLstatIfPossible/TestSymlinkIfPossible/TestReadlinkIfPossible, which fail identically on unmodified master in my environment (Windows, no privilege to create symlinks) — confirmed pre-existing and unrelated by stashing my changes and rerunning.

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