[material-ui] Text inputs render with invalid type="string" attribute

[dstaley]

Prerequisites

• I have read the documentation;
• In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.
• Ideally, I'm providing a sample JSFiddle or a shared playground link demonstrating the issue.

Description

When using the material-ui theme, text inputs render with their type attribute incorrectly set to string instead of text.

Steps to Reproduce

1. Playground link
2. Inspect either the first name or last name text fields.

Expected behavior

Inputs are rendered with type="text"

Actual behavior

Inputs are rendered with type="string"

Version

master