Resolves #140: substitute placeholders that resolve to true to their key name

Warning: this change breaks the undocumented (and so, unsupported) case in which a key resolving to a boolean true, was actually intended to generate a 'true' permalink path part.

Author: webketje

README.md

@@ -74,7 +74,9 @@ Every `permalinks()` instantiation supports the following options:
 - duplicates - what to do when 2 files have the same target destination. See [Ensure files have unique URI's](#ensure-files-have-unique-uris)
 - linksets, see [Defining linksets](#defining-linksets)
 
-Placeholder substitution will always `toString` the value. For example, when you have an `:array` placeholder and a file with front-matter `array: ['one','two']`, it will substitute into `'onetwo'`, but you can refer to the n<sup>th</sup> value with a dot-delimited keypath (eg `:array.0`). A boolean `false` will result in an error unless the placeholder is optional (it would then be an empty string = omitted), a boolean `true` will result in the string `'true'`.
+Placeholder substitution will mostly `toString` the value. For example, when you have an `:array` placeholder and a file with front-matter `array: ['one','two']`, it will substitute into `'onetwo'`, but you can refer to the n<sup>th</sup> value with a dot-delimited keypath (eg `:array.0`).
+
+A boolean `false` will result in an error unless the placeholder is optional (it would then be an empty string = omitted), **but a boolean `true` will see the placeholder substituted with its key name** (eg `:placeholder` for a file with front-matter `placeholder: true` will become `placeholder`).
 
 ### Matching files
 

src/index.js

@@ -208,7 +208,7 @@ const replace = ({ pattern, ...options }, data) => {
     } else if (key === 'dirname') {
       ret[key] = val
     } else {
-      ret[key] = options.slug(val.toString())
+      ret[key] = options.slug((typeof val === 'boolean' ? key : val).toString())
     }
   }
 

test/fixtures/booleans/expected/blog/how-to/use-metalsmith-permalinks/index.html

@@ -0,0 +1,6 @@
+---
+blog: true
+news: true
+permalink: im-fixed
+category: fixed
+---

test/fixtures/booleans/expected/im-fixed/index.html

@@ -0,0 +1,5 @@
+---
+blog: false
+news: true
+category: general
+---

test/fixtures/booleans/expected/news/general/new-permalinks-release/index.html

@@ -0,0 +1,5 @@
+---
+blog: true
+news: false
+category: how-to
+---

test/fixtures/booleans/src/fixed-permalink.html

@@ -47,6 +47,13 @@ const fixtures = [
     folder: 'shorthand',
     options: ':title?'
   },
+  {
+    message: 'should substitute booleans for their key name',
+    folder: 'booleans',
+    options: {
+      pattern: ':blog?/:news?/:category/:basename'
+    }
+  },
   {
     message: 'should format a date',
     folder: 'date',