message: Fix default font weight & size in Heading
diff: import styled from 'styled-components'
  import PropTypes from 'prop-types'
  -import {TYPOGRAPHY, COMMON, Base} from './constants'
  +import {TYPOGRAPHY, COMMON, Base, get} from './constants'
  import theme from './theme'
  const Heading = styled(Base)`
  + font-weight: ${get('fontWeights.bold')};
  + font-size: ${get('fontSizes.5')};
  ${TYPOGRAPHY} ${COMMON};
  `
  Heading.defaultProps = {
  theme,
  m: 0,
  - fontSize: 5,
  is: 'h1'
  }
  