String type must be strict.

It makes sense to coerce from more general -> more specific data types.
eg. string -> int, etc. But coercing the other way is problematic
in the case of chaining because the more general type will swallow the
specific type. eg. Setting `false` on type: [‘string’, ‘boolean’] will 
coerce the boolean to a string, and will never allow the value to be
a boolean.

spec/config-spec.coffee

@@ -701,21 +701,21 @@ describe "Config", ->
         atom.config.set('foo.bar.aString', 'yep')
         expect(atom.config.get('foo.bar.aString')).toBe 'yep'
 
-      it 'will only set strings, numbers and booleans', ->
-        expect(atom.config.set('foo.bar.aString', 123)).toBe true
-        expect(atom.config.get('foo.bar.aString')).toBe '123'
+      it 'will only set strings', ->
+        expect(atom.config.set('foo.bar.aString', 123)).toBe false
+        expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
         expect(atom.config.set('foo.bar.aString', true)).toBe false
-        expect(atom.config.get('foo.bar.aString')).toBe '123'
+        expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
         expect(atom.config.set('foo.bar.aString', null)).toBe false
-        expect(atom.config.get('foo.bar.aString')).toBe '123'
+        expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
         expect(atom.config.set('foo.bar.aString', [])).toBe false
-        expect(atom.config.get('foo.bar.aString')).toBe '123'
+        expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
         expect(atom.config.set('foo.bar.aString', nope: 'nope')).toBe false
-        expect(atom.config.get('foo.bar.aString')).toBe '123'
+        expect(atom.config.get('foo.bar.aString')).toBe 'ok'
 
     describe 'when the value has an "object" type', ->
       beforeEach ->

src/config.coffee

@@ -667,12 +667,10 @@ Config.addSchemaEnforcers
           throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a boolean or the string 'true' or 'false'")
 
   'string':
-    coerce: (keyPath, value, schema) ->
-      switch typeof value
-        when 'number', 'string'
-          value.toString()
-        else
-          throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a string or number")
+    validate: (keyPath, value, schema) ->
+      unless typeof value is 'string'
+        throw new Error("Validation failed at #{keyPath}, #{JSON.stringify(value)} must be a string")
+      value
 
   'null':
     # null sort of isnt supported. It will just unset in this case